Manufacturers of zinc batteries and fuel cells are constantly seeking improvement in the performance of these electrochemical energy devices. There is an ever-increasing demand for batteries that can provide higher power without unacceptable sacrifices in desirable battery performance characteristics, such as long discharge life (high capacity), long storage life, resistance to electrolyte leakage and ease of manufacturing. There has been continuous effort in the industry to develop better zinc particulate material for alkaline-zinc batteries, zinc-air batteries and fuel cells. Attempts have been made to improve the particle size distribution, to change alloy composition, or to use different forms of particulates such as zinc ribbons, or flakes, or wires. Attempts have also been made to increase the interface area between the anode and the cathode.
Zinc materials for alkaline battery anodes have been constructed in three basic dimensional forms, namely zero-dimensional (where the x, y, z dimensions are about the same) such as with spheres, cubes, or one-dimensional (where one of the three dimensions is significantly larger than the other two dimensions) such as fibers and wires, and two-dimensional (where one dimension is significantly smaller than the other two dimensions) such as sheets. Zinc powder, as zero-dimensional material (produced by atomization or electroplating), zinc fibers as one-dimensional material (including needles, wools, wires and filaments which can be produced by different methods including extruding, milling, casting, and electroplating), and zinc sheets as two-dimensional material (including solid discs, flakes, ribbons and sheets, and perforated and expanded sheets which can be produced with different methods including rolling and casting) have been developed in the industry. In order to obtain the distributions of solid and porous space required for zinc anodes to perform satisfactorily in batteries or fuel cells, different methods can be utilized to process the materials of the three different dimensional types.
From a system perspective, the dimension, the shape of the anode and cathode, the anode/cathode interface area, the separator material, the amount and composition of electrolyte, the material and design of current collector, and the properties of electrodes are all highly important in the performance of batteries and fuel cells. These need to be properly designed. With respect to the properties of electrodes, important parameters include specific surface area (defined as the total physical surface area per unit of weight, m2/g), effective surface area (the amount of surface area that is electrochemically active), surface activity, porosity, electrical conductivity, and mechanical stability. Many of these characteristics are determined by the distributions of solid and porous space in the given volume of the space that is determined by the design of the battery and the characteristics of the materials.
Different forms of materials have different value sets for specific surface area, effective surface area, surface activity, porosity, electrical conductivity, and mechanical stability. A zinc powder that performs well in a battery of given design is the result of the combined effect of these parameters. Control of only one or two of these parameters alone will not result in an anode that can perform well in batteries. For example, for the same specific surface area, powders of different particle distribution can have different discharging performance as is disclosed in U.S. Pat. No. 6,284,410 A1. A powder that performs well in one battery design may not perform well in another battery design. For this reason, there has been continuous development of zinc powders for use in different alkaline and zinc-air batteries.
Control of the effective surface area of electrode materials is critically important in battery design. In general, the smaller the particle size, the larger the surface area. A large surface area will result in high activity and thus high battery power. However, a zinc anode with a very large surface area has a downside because it can cause excessive gassing due to the corrosion of the zinc in the electrolyte. The zinc powders used in making alkaline batteries in the market place have a specific range of surface area, typically between 0.01 and 0.1 m2/g. There has been a trend to use finer zinc particulates, i.e. large surface area, in order to increase the power of alkaline batteries as demonstrated in a recent U.S. Pat. No. 6,521,378.
Control of porosity (the space between zinc particulates that is occupied by the electrolyte) of the zinc anode is another important consideration. For a given battery design, there is a certain range of preferred porosity, and anodes made of a specific form of zinc particulate material perform the best in these conditions. In alkaline batteries using atomized zinc powder, the lowest volume percent in the zinc anodes is not less than 28% of the zinc anode paste (including zinc powder, KOH, additives and gelling agents) in order to match the positive electrode's rate of electrochemical output and provide sufficient particle-to-particle and particle-to-current collector contact to maintain the zinc anode's electrical conductance (see, for example, U.S. Pat. No. 6,221,527). Below this amount, voltage instability occurs and battery performance becomes sensitive to shock and vibration. On the other hand, if the zinc powder volume in the anode is too high, e.g. higher than 50%, there is not sufficient space between the particles for storing electrolyte and the dissolution product which is a mix of zinc oxide, hydroxide and some elemental zinc.
The zinc powder used in alkaline battery applications before being mixed with electrolyte typically has a density of 3 to 3.5 g/cm3, which is about 42% to 50% zinc volume and 50% to 58% in porosity. To reach the porosity required for alkaline cells to perform well, which is typically about 70%, manufacturers typically use a gelling agent to make the mix of zinc powder and electrolyte such that the zinc particles are not densely packed but are somewhat suspended in the electrolyte gel. However, for a given powder (certain shapes of particles and particle size distributions) there is only a narrow range that the particles can be suspended without losing electrical contact between the particles.
Surface activity determines the rate of discharge under a given performance condition as well as the gassing rate under an idle condition. The surface activity of zinc is related to a set of electrochemical properties, which are well documented by Zhang (Corrosion and Electrochemistry of Zinc, Plenum Publishers, 1996) the contents of which are incorporated herein by reference. It is desirable for batteries designed for high drain applications to have materials that have a surface activity that allows a high rate of discharge but have a low rate of gassing. Commercially available zinc powders used in alkaline batteries and fuel cells are typically alloyed to various compositions in order to obtain a desirable surface activity. In addition to alloying, surface activity may also be controlled by adding corrosion inhibitors in the electrolyte, surface treatment of zinc particulates, and the like.
Physical stability of the anode is another consideration in battery design. Atomized powder, as a physical body, generally behaves like a liquid, and is not in a self-contained form. It tends to flow under the effect of gravity. The use of gel serves to help immobilize the particles relative to each other and to the current collector and thus provides a certain mechanical stability to the anode, which is essential for reliable battery performance.
There is a combined effect of the above mentioned parameters on zinc anodes made from atomized powder in order to have desirable performance of the batteries in specific applications. For anodes made from atomized powder, gelling is required to provide the porosity required for desirable performance of the batteries. However, since gelling causes reduction in particle connectivity and thus anode conductivity, the amount of gelling and thus the extent of porosity control is limited. Furthermore, although gelling provides certain immobilization of zinc powder particles, the gelled anode is still a paste and does not have the same mechanical integrity as a solid. Thus, battery anodes made from atomized powder do not achieve optimal utilization of the energy that is latent in the zinc anode because they do not allow for independent control over porosity and surface area while maintaining good electrical conductivity among the zinc particles.
Zinc powder, as a zero-dimensional material, is typically produced by an atomizing process and has been the dominant material for making anodes for alkaline and zinc-air batteries that are available in the market place. Zinc powder requires gelling to make anode that can perform in battery. However, a gelled anode has lower electrical conductivity compared to non-gelled material and has limited mechanical integrity and a limited range of control over porosity without losing electrical contact between the particles. Thus, anodes made of atomized powder do not allow the optimal utilization of the latent energy in the zinc metal. Much prior art can be found in the patent literature and new patents are being granted on a continuous basis. For example, U.S. Pat. No. 6,521,378 B2 discloses the making of zinc anodes using powders that have a multi-modal distribution of zinc-based particles. WO 01/56098 A2 discloses the making of zinc particle agglomerates using low melting metal binder to make agglomerates of powder particles. U.S. Pat. No. 6,284,410 and U.S. Publication No. 2003/0203281 A1 show related control of particle distribution of atomized powder to improve performance under high discharge rate.
Zinc powder produced through electroplating in KOH electrolyte has been used for making anodes in battery and fuel cell applications. The powder, usually in the form of dendrites, generally has a very high specific surface area. Alkaline Zn—Ag batteries using electroplated powder have been used in many military applications such as power sources for launching missiles and rockets [Zinc-Silver Oxide Batteries, Ed. By Flecher & Lander, 1971, John Wiley & Sons]. More recently, electroplated zinc dendritic material has found application in power sources for vehicles. U.S. Pat. Nos. 5,599,637 and 5,206,096, for example, teaches the art of using electroplated zinc dendrite material to make anodes for mechanically refuelable zinc-air fuel cells to power electric buses. A similar type of material is used to make anodes for mechanically refuelable zinc-air fuel cells for power electric bicycles (see web page of Powerzinc Inc). In mechanically refuelable batteries and fuel cells, the discharged zinc anodes are physically removed from the cells and replaced by fresh anodes.
However, the high surface area also means a high corrosion rate of the anode in KOH solution and thus the anode is not suitable for long-term storage. Also, the anode is made by pressing dendritic zinc into a plate which has a limited strength and tends to break under mechanical stress, though it is in solid form. Furthermore, the use of electroplating to produce the anode also has a limitation in economical operation of mechanically refuelable zinc-air fuel cells.
One-dimensional forms of materials such as fibers, needles, wools or ribbons for making battery anodes have been documented as early as 1974 [Kordesch, Batteries, Volume 1 Manganese Dioxide]. The use of fibrous materials for making anodes allows the independent control of surface area and porosity over a wide range while maintaining electrical connectivity of all zinc fibers with no need for a gelling agent. The advantage of the solid electrode made of fibrous materials is that individual fibers are physically connected with a number of other fibers and are linked together as a solid form. The density and porosity is then controlled by the degree of confinement of the fibrous materials under a mechanical pressing.
There have been a number of prior disclosures about using zinc in a one-dimensional form of fibers, needles, wools or ribbons as battery anodes. For example, U.S. Pat. No. 3,853,625 discloses the use of zinc filaments produced by electroplating for making battery anodes. U.S. Pat. No. 5,584,109 discloses an electrode made with caddied and extruded metallic fibers. U.S. Pat. No. 6,221,527 B1 discloses zinc ribbon for use as battery anodes. U.S. Publication No. 2002/0142202 A1 discloses an electrode for an electrochemical cell. The electrode comprises a plurality of fibers comprised of an electrically conductive material configured to conduct electrons to an electrolyte of the electrochemical cell. U.S. Publication No. 2003/0170543 A1 discloses the use of fibers produced by mechanical milling for making zinc battery anodes. While the disclosures in these patents and publications demonstrate one-dimensional forms, they neither demonstrate the utility of the anode made of these materials in actual batteries nor do they show materials, conditions and processing procedures that are practically usable.
Some prior art also discloses the use of two-dimensional forms (sheets, flakes etc.) of zinc materials for making anodes. For example, U.S. Pat. No. 4,226,920 teaches the making of anode by rolling expanded and woven metal zinc mesh to the final cylindrical form and size. The discharging performance of such anodes, although showing benefits compared to the conventional cells at the time, is nonetheless greatly inferior to the performance of current commercial batteries. WO 01/24292 A1 discloses rounded zinc flakes coated with a gelling agent for making battery anodes. The disclosure is not commercially practical because the processing steps for making such anodes are complex and thus costly. U.S. Pat. No. 6,673,494 discloses the use of expanded mesh to make anodes but provides no indication whether such anodes can provide improved performance. U.S. Pat. No. 6,022,639 discloses the use of zinc flakes for making high performance batteries. Nonetheless, the use of gel to control porosity and mechanical integrity of the anode is still required.
Flake and sheet materials have a shortcoming related to effective surface area, which is the surface area available for electrochemical reaction. Electrochemical reactions become stagnated whenever the path for current flow is narrowed as is the case in the gaps between overlapped sheets of metals. Because there is a tendency for individual sheets and flakes to overlap with each other, a significant percentage of surface area becomes unavailable for reaction with these materials. Thus, the effective surface area of such material is generally much smaller than the specific surface area of the flake or sheet material.